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Periodic Table--Helium

There are two stable isotopes of He, 3He (1.37 x 10-4 %) and 4He (>99.99 %). Although ground waters contain 3He from several sources other than 3H decay (see below), determination of both 3H and tritogenic 3He (3He*) can be used as a dating tool (Tolstikhin and Kamensky, 1969; Torgersen et al., 1977; Takaoka and Mizutani, 1987; Schlosser et al., 1988). While decay of 3H to 3He commences in the unsaturated zone, dissolved 3He generated from 3H decay can be lost to the atmosphere. It is not until it enters the ground water that 3He is preserved because gas transport in the unsaturated zone is rapid relative to transport in ground water. Once isolated from the atmosphere below the water table, dissolved 3He concentrations will rise. For this reason, a 3H/3He age of t = 0 should occur at the water table, and ages should increase with depth as ground water becomes older. In the case of a fluctuating water table, t = 0 is set at the seasonal low water table position (Solomon et al., 1993).

The tritogenic 3He component is calculated by subtracting from the measured value that due to atmospheric solubility (a function of recharge temperature), excess air and radiogenic production (by decay of U/Th-series elements). These different sources of 3He are identified by measuring concentrations of 4He and other noble gases (Torgersen et al., 1977, 1979; Weise and Moser, 1987; Schlosser et al., 1989).

3H/3He ages accurately reflect hydraulic ages (subject to the uncertainties discussed above) in ground-water systems with piston flow and negligible mixing. However, the concentration ratio of 3H and 3He at any given point will not always accurately reflect the ground-water age because the concentration gradients are usually different and the diffiusion coefficient for 3He > 3H in water. Underestimates of ages of surface water bodies have been shown to be the result of diffusive loss of 3He to the atmosphere (Wuest et al., 1992), and will similarly lead to underestimates of ground-water ages (Schlosser et al., 1989). Analytical uncertainties usually result in errors in age estimates of less than 10% (Solomon et al., 1993). In addition to dating waters, 3He levels have also been shown to be a function of the vertical-flow velocity (recharge rate) and dispersivity (Schlosser et al., 1989).

Radiogenic stable 4He has been used to locate areas of deep ground water discharge. High 4He concentrations are common in deep ground waters as a result of subsurface a-decay of natural U- and Th-series elements in rocks and sediments. Mass transfer between aquifer solids and ground water occurs, resulting in a relationship between ground-water travel time (or contact time) and the 4He concentration in ground water. If the 4He solid-to-liquid mass transfer rate is known, it is possible to use 4He as a quantitative tracer of ground-water travel times (e.g. Marine, 1979; Andrews and Lee, 1979; Torgersen, 1980; Stue et al., 1992). If the solid-to-liquid mass transfer rate is governed by the radioactive decay of U and Th, significant (i.e. measurable) amounts of radiogenic 4He will exist in ground water after a contact time on the order of 1000 years and can theoretically continue to accumulate for millions of years. Thus, 4He has generally been considered to be a hydrologic tracer over time scales of 103 to 106years (e.g. Mazor and Bosch, 1992); however, a recent study by Solomon et al. (1996) has shown that some aquifer solids are currently losing ancient 4He (previously trapped within crystal lattices) at rates that are controlled by solid state diffusion. If quantified, such high solid-to-liquid mass transfer rates allow ground-water dating with 4He for water as young as 10 years and possibly as old as 103 years.

Ground water ages using 4He are calculated by determining the solid-to- liquid mass transfer rate, and then measuring the 4He content of ground-water samples. The solid-to-liquid mass transfer rate can be determined directly in the laboratory or by calibration using ground-water age data obtained using other (i.e. CFCs or 3H/3He) dating methods. Once the solid-to-liquid mass transfer rate has been estimated, ground-water ages are determined by measuring total He and Ne in ground-water samples, and computing the component of radiogenic 4He. Because ground water contains 4He from the atmosphere as well as that released from solid phases, it is necessary to separate radiogenic from atmospheric 4He.

The uncertainty in ground-water ages determined using radiogenic 4He results mostly from uncertainty in the solid-to-liquid mass transfer rate. While the age uncertainty using 4He is much greater than with CFCs, 3H/3He, or 85Kr, the potential for dating waters in the range of 50 to 1000 (where no other dating methods are practical) makes the use of radiogenic 4He attractive.

Further information can be found in Clark and Fritz (1997), Environmental Isotopes in Hydrology, (CRC Press):

Source of text: This review was assembled by Eric Caldwell and Dan Snyder, primarily drawing from Solomon et al., 1998.

Andrews, J. N., and Lee, D. J. (1979). "Inert gases in groundwater from the Bunter Sandstone of England as indicators of age and paleoclimatic trends." J. Hydrol., 41: 233-252.
International Union of Pure and Applied Chemistry (1991). "Isotopic compositions of the elements", Pure & Appl. Chem., 63, 7:991-1002.
Marine, I. W. (1979). "The use of naturally occurring helium to estimate groundwater velocities for studies of geologic storage of radioactive waste." Water Resour. Res., 15: 1130-1136.
Mazor, E. and Bosch, A. (1992). "Helium as a semi-quantitative tool for groundwater dating in the range of 104 to 108 years." In: Consultants meeting on isotopes of noble gases as tracers in environmental studies. IAEA, Vienna, 305 p.
Plummer, L. N., Michel, R. L., Thurman, E. M., and Glynn, P. D. (1993). "Environmental tracers for age dating young ground water", in W. M. Alley (Ed.), Regional Ground-Water Quality , Van Nostrand Reinhold, New York, pp. 255-293.
Schlosser, P., Stute, M., Dorr, H., Sonntag, C. and Oto, K. M. (1988). "Tritium/3He dating of shallow groundwater." Earth Planet. Sci. Lett., 89: 353-362.
Schlosser, P., Stute, M., Dorr, H., Sonntag, C., and Munnich, K. O. (1989). "Tritogenic 3He in shallow groundwater". Earth Planet. Sci. Lett., 94: 245-254.
Solomon, D. K., Cook, P. G., and Sanford, W. E. (1997). "Dissolved Gases in Subsurface Hydrology." In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, pp. 291-318.
Solomon, D. K., Hunt, A., and Poreda, R. J. (1996). "Source of radiogenic helium 4 in shallow aquifers: Impications for dating young groundwater." Water Resour. Res., 32: 1805-1813.
Solomon, D. K., Poreda, R. J., Cook, P. G. and Hunt, A. (1995). "Site characterization using 3H/3He ground water ages, Cape Cod, MA." Ground Water, 33: 988-996.
Solomon, D. K., Schiff, S. L., Poreda, R. J. and Clarke, W. B. (1993). "A validation of the 3H/3He method for determining groundwater recharge." Water Resour. Res., 29, 9: 2851-2962.
Stute, M., Sonntag, C., Deak, J. and Schlosser, P. (1992). "Helium in deep circulating groundwater in the Great Hungarian Plain: flow dynamics and crustal and mantle helium fluxes." Geochim. et Cosmochim. Acta, 56: 2051-2067.
Takaoka, N. and Mizutani, Y. (1987). "Tritogenic 3He in groundwater in Takaoka." Earth Planet. Sci. Lett., 85: 74.
Tolstikhin, I. N. and Kamensky, I. L. (1969). "Determination of groundwater age by the T-3He method." Geochem. Int., 6, 810-811.
Torgersen, T., Zop, T., Clarke, W. B., Jenkins, W. J. and Broeker, W. S. (1977). "A new method for physical limnology - tritium-helium-3 ages - results for Lakes Erie, Huron and Ontario. Limnol. Oceanog., 22: 181-193.
Torgersen, T. (1980). "Controls on porefluid concentrations of 4He, 222Rn and the calculation of 4He/222Rn ages." J. Geochem. Explor., 13, 57-75.
Torgersen, T. (1990). "Helium-4 model ages for pore fluids from fractured lithologies", in Isotopes of Noble Gases as Tracers in Environmental Studies; Proceedings of a Consultants Meeting, International Atomic Energy Agency, Vienna. pp. 179-201.
Weise, S. and Moser, H. (1987). "Groundwater dating with helium isotopes." In: Isotope Techniques in Water Resource Development." IAEA, Vienna, pp. 105-126.
Zoback, M. D., Silver, L. T., Henyey, T., Thatcher, W. (1988). "The Cajon Pass scientific drilling experiment: Overview of Phase 1." Geophys. Res. Lett., 15, pp. 933- 937.
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Periodic Table
Fundamentals of Stable Isotope Geochemistry
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